1. Field of the Invention
The present invention relates to a sheet supply apparatus for supplying a sheet such as an original, a recording medium and the like to an image forming apparatus such as a printer, a facsimile, a copying machine and the like.
2. Related Background Art
Among sheet supply apparatuses used with an image forming apparatus, there has been proposed a sheet supply apparatus in which a sheet support plate for supporting a sheet stack can be lifted and lowered so that, when the sheet support plate is lifted, the sheet stack is urged against a sheet supply roller and an uppermost sheet in the sheet stack is supplied by rotation of the sheet supply roller and that, after the sheet is supplied, the sheet support plate is lowered.
An example of such a conventional sheet supply apparatus will now be explained with reference to FIGS. 13 and 14. Incidentally, FIG. 13 is a sectional view of a sheet supply apparatus, and FIG. 14 is a view showing a drive system for lifting and lowering a sheet support plate.
A sheet support plate 100 for stacking sheets is pivotally mounted on a rock shaft 102 for upward and downward rocking movement and is normally biased upwardly by pressure springs 104. Cams 108 are provided on a shaft 106a for supporting a sheet supply roller 106 so that, when the sheet supply roller 106 is stopped, the cams 108 are contacted with rollers 110 provided on the sheet support plate 100 to lower and maintain the sheet support plate in a lowered position, i.e. a sheet stacking position where any sheets can be loaded on the sheet support plate.
A notched gear 112 having a non-toothed portion 112a is secured to the shaft 106a of the sheet supply roller 106, which gear is adapted to engaged by an idler gear 114 connected to a drive source (not shown). Rotation of the notched gear 112 can be regulated by a rock rod 116. When the rotation of the notched gear is regulated, the non-toothed portion 112a of the notched gear 112 is opposed to the idler gear 114, with the result that the driving force from the drive source is not transmitted to the notched gear.
Further, there is provided a release mechanism for releasing the regulation of the rod 116. The release mechanism comprises a release cam 118 for rocking the rod 116 to release the regulation of the rod 116 by rotation of the cam, a notched gear 120 disposed in coaxial with the release cam 118 and adapted to be engaged by the idler gear 114, a spring 122 for biasing the notched gear 120 toward its rotational direction, and a solenoid 124 for regulating a position of the release cam 118 in opposition to a biasing force of the spring 122.
On the other hand, a separation means for separating the sheets is disposed at a downstream side of the sheet support plate 100. The separation means comprises a main pad 126 and a sub pad 128. The main pad 126 adapted to be urged against the sheet supply roller 106 in order to separate the sheets one by one when the sheets are fed out by the sheet supply roller 106, and is contacted with and separate from the sheet supply roller 106 in response to the lifting and lowering movements of the sheet support plate 100. The sub pad 128 is advanced or protruded into a sheet path when the main pad 126 is separated from the sheet supply roller 106, with the result that tip ends of second, third and other sheets are stopped by the sub pad, thereby preventing the double-feed of sheets.
With this arrangement, when the regulation of the release cam 118 is released by activating the solenoid 124, the notched gear 120 is rotated by the biasing force of the spring 122 to engage the notched gear by the idler gear 114. As a result, the release cam 118 is rotated. When the rod 116 is rocked by the rotation of the cam 118 to release the regulation of the cams 108, the cams 108 are rotated by the biasing forces of the pressure springs 104 via the rollers 110. Consequently, the sheet support plate 100 is lifted, and, at the same time, the notched gear 112 is engaged by the idler gear 114, with the result that the driving force is transmitted to the shaft 106a of the sheect supply roller 106 to rotate the sheet supply roller 106, thereby the uppermost sheet in the sheets stacked on the sheet support plate 100 is fed out. When the sheet supply roller 106 is rotated by one revolution, the sheet support plate 100 is lowered again by the cams 108, and, in the lowered position, the rotation of the cams 108 is regulated by the rod 116.
While the sheets are being separated one by one between the sheet supply roller 106 and the main pad 126 by the rotation of the sheet supply roller, the cams 108 start to lower the sheet support plate 100 again. In this case, although the main pad 126 starts to separate from the sheet supply roller 106, before the separation of the main pad, the sub pad 128 is protruded into the sheet path. The sub pad 128 serves to prevent the second and other sheets from being fed out together with the separated sheet. In this way, back tension acting on the fed sheet can be reduced and the driving force for conveying the sheet can be set smaller, with the result that the apparatus can be made compact and cost-down can be achieved.
However, the above-mentioned conventional sheet supply apparatus has the following drawback.
Namely, in the above-mentioned conventional sheet supply apparatus, since one revolution of the sheet supply roller 106 is controlled by using the notched gear 112 and the sheet support plate 100 is lifted and lowered on the basis of one revolution of the sheet supply roller, a sheet feeding amount of the sheet supply roller 106 is limited. That is to say, since the sheet is fed only during one revolution of the sheet supply roller 106, a sheet feed amount is relatively small, and, thus, a distance between the sheet supply roller and convey rollers disposed at a downstream side of the sheet supply roller must be shortened. As a result, the number of convey rollers must be increased accordingly, which results in the cost-up of the apparatus.
Although the sheet feed amount can be increased by increasing a diameter of the sheet supply roller, the apparatus will become bulky.